1. Field of the Invention
This invention generally relates to ceramic electronic packaging. Specifically, there is a multilayered low temperature co-fired ceramic assembly (LTCC) with a constraining core to minimize shrinkage of outer ceramic layers during firing. The ceramic layers have different dielectric constants to allow fabrication of high density capacitors and other electronic components.
2 Description of Related Art
Various devices are well known for providing ceramic packages for semiconductor devices and passive components. One of the prior art designs is a low temperature co-fired ceramic (LTCC) substrate. The LTCC ceramic is made of layers of ceramic material, which in an unfired state, are called green tapes. Circuit lines, resistors, capacitors, bonding pads and vias are created on the surface and in holes of the green tapes by conventional thick film screening techniques. The layers are stacked on top of each other laminated and fired at a relatively low temperature in a furnace. During firing, the LTCC shrinks along the x, y and z axes typically 10-25 percent depending upon the LTCC formulation.
Despite the advantages of the prior art LTCC designs, problems occur with the registration or alignment of the circuit lines and components on the exterior surfaces during manufacturing. During firing, the shrinkage of the LTCC causes the external features to vary with respect to true position. This true position error can cause misalignment when attaching components or printing post-fire materials, resulting in a defective part that is non-repairable and has to be discarded.
Another problem with LTCC electronic packages occurs in the fabrication of buried capacitors within the package. It is desirable to have a high dielectric constant between capacitor electrodes so that a given capacitance can be achieved without large electrodes. At the same time, it is desirable for the circuit lines that attach to the capacitor electrodes to be located on a low dielectric constant substrate to reduce unwanted parasitic effects such as coupling to other lines or embedded components.
Several attempts have been made in the prior art to solve some of these problems. U.S. Pat. No. 5,518,969, shows a process for producing low shrink ceramic compositions. U.S. Pat. No. 5,144,526, shows a low temperature co-fired ceramic structure containing buried capacitors. U.S. Pat. No. 5,745,334, shows a capacitor formed within a printed circuit board. None of these patents have been able to overcome all of the problems of the prior art.
It is a feature of the invention to provide a low temperature co-fired ceramic assembly (LTCC) with a constraining core of differing dielectric constants to minimize shrinkage of outer ceramic layers during firing.
A further feature of the invention is to provide a multilayered low temperature co-fired ceramic assembly including a planar ceramic core. The core has a first ceramic layer with a first dielectric constant and a second ceramic layer adjacent to the first ceramic layer. The second ceramic layer has a second dielectric constant. A third ceramic layer has a third dielectric constant. A fourth ceramic layer has a fourth dielectric constant. The ceramic core is located between the third and the fourth ceramic layers. Several electrically conductive vias extend through the first, second, third and fourth ceramic layers. Several circuit features are located on the first, second, third and fourth ceramic layers. The vias electrically connect the circuit features on the layers.